I actually looked around for turboprops in a pusher configuration, and so far I only found one: P180 Avanti. You can see the propellers are behind the turbine, and in fact it looks like two exhaust ports lead right into the propellers.

How is this possible? Turbine exhaust is very, very hot. "Superheated steam" is actually a technically accurate term. Won't this disturb the propellers and cause them to fail or else need huge maintenance? Is there some elegant way to avoid this geometry in a pusher configuration?

$\begingroup$Turbine exhaust was in fact a problem on one airplane, although not affecting the props. The DeHavilland -7's overwing exhaust was found to be actually annealing the 7075 alloy upper wing planks and they were being found wavy in service from very slow compression buckling. Dash 7s all have skin doublers behind the nacelles now.$\endgroup$
– John KMay 31 '18 at 13:56

3 Answers
3

Because the blade is moving in and out of the exhaust stream at high velocity, and the exhaust stream is perhaps 20% of the total exposure, the heat transfer to the blades is negligible from the standpoint of heating up aluminum enough to affect its heat treat, or heating epoxy to its transition temperature. Meanwhile there are the anti-icing benefits.

There is an issue with the exhaust impinging on the prop on the Avanti, and there is a Hartzel Service Bulletin http://hartzellprop.com/wp-content/uploads/SB181A-R06-W.pdf that covers it. The carbon and heat from the exhaust attacks the aluminum of the blade and the problem is blade corrosion once the paint finish starts to erode. The SB is to inspect, clean up, and repaint the blades from this damage. (Carbon and aluminum are at opposite ends of the galvanic scale and don't like to live together; this caused much grief on the CRJ program when someone who evidently missed their corrosion module in university decided to put carbon floor panels on aluminum support beams without an adequate barrier, with unfortunate results and driving a switch to titanium beams).

Really though, with the Avanti the biggest problem is the prop directly behind the gear acting like a FOD catcher for stuff thrown up by the tires. You wouldn't want to try to fly an Avanti off a gravel strip, that's for sure.

$\begingroup$Anti-icing is cool, but do you have a source for negligible heat transfer in terms of structural weakening? I would think it depends on how many blades and how thick they are. I've seen some really thick "fan" props, like on the Airbus A400M (although that's not pusher).$\endgroup$
– DrZ214May 31 '18 at 14:51

$\begingroup$I'm going by the fact that the configuration was certified and in certification the margins of error are very large. That being said, the potential is there and I'm starting to dimly remember some kind of issue with a pusher prop being damaged by exhaust heat, but can't recall the details. I'll have to hunt around a bit.$\endgroup$
– John KMay 31 '18 at 16:09

$\begingroup$Just remembered and edited my post. Corrosion is the bugaboo, not annealing.$\endgroup$
– John KMay 31 '18 at 17:06

$\begingroup$Any idea how long before corrosion becomes significant? I saw in the PDF it recommended 200 flight hours between inspections, but that's just inspections, not repairs. However, if repairs are just re-applying some paint, it doesn't sound that bad. BTW, what is CRJ program?$\endgroup$
– DrZ214May 31 '18 at 17:17

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$\begingroup$For structural and corrosion problems, there is usually a 3x safety factor applied to test results. Piaggio and Hartzel would have done some corrosion testing to simulate the in-service condition, and/or collected service data, and discovered that significant corrosion was happening in as little as 600 hours, so that would drive an inspection interval of 200.$\endgroup$
– John KMay 31 '18 at 17:48

I suppose one could route an exhaust stack from the engine installation to a location away from the engine or propellers. But then again the benefits would have to outweigh the drawbacks of such a design.

Adding additional exhaust routing would solve the problems of structural damage due to heating, But it’s also bulkier, heavier and imposes additional design requirements for heat shielding the aircraft structure, preventing exhaust gas leaks and designing an exhaust outlet installation to vent the exhaust gases away from the airframe somewhere else on the aircraft. And that’s not really necessary if the temperature from the exhaust gases around the propeller blades is sufficiently cool enough to not cause structural damage the blades. A PT6 usually has a maximum Inter-turbine temperature (ITT) around 750° C but the temperature of the exhaust gases exiting the exhaust stubs Is going to be lower-probably only order of 400°C or so. Add intermixing with outside air flow and the temperature of the exhaust gases may be down as low as 250 to 300°C by the time it passes through the propeller arc. This is going to be far from hot enough to damage the propeller blades provided their manufactured out of the right material to the right specifications. It has an additional bonus and that the hot exhaust gases can serve as a means to prevent ice accumulation on the propeller blades during flight, thereby forgoing a requirement for hot props or another kind of prop anti ice solution

The exterior noise level and its higher pitched sound has been exposed
and presented to be the result primarily of the interaction of the
turbine engine exhaust flows and the five-bladed pusher propellers.